Scaffold-organized metal, alloy, semiconductor and/or magnetic clusters and electronic devices made using such clusters

ABSTRACT

A method for forming arrays of metal, alloy, semiconductor or magnetic clusters is described. The method comprises placing a scaffold on a substrate, the scaffold comprising molecules selected from the group consisting of polynucleotides, polypeptides, and perhaps combinations thereof. Polypeptides capable of forming α helices are currently preferred for forming scaffolds. Arrays are then formed by contacting the scaffold with plural, monodispersed ligand-stabilized clusters. Each cluster, prior to contacting the scaffold, includes plural exchangeable ligands bonded thereto. If the clusters are metal clusters, then the metal preferably is selected from the group consisting of Ag, Au, Pt, Pd and mixtures thereof. A currently preferred metal is gold, and a currently preferred metal cluster is Au 55  having a radius of from about 0.7 to about 1 nm. Compositions also are described, one use for which is in the formation of cluster arrays. One embodiment of the composition comprises plural monodispersed, ligand-stabilized clusters coupled to a polypeptide.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of pending U.S. patentapplication Ser. No. 10/013,334, filed Nov. 5, 2001, which is acontinuation of U.S. patent application Ser. No. 09/085,390, filed May27, 1998, now abandoned, which claims the benefit of the earlier filingdate of U.S. Provisional Patent Application No. 60/047,804, filed May27, 1997, now abandoned. Each of these prior patent applications isincorporated herein by reference.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

[0002] This invention was made in part using funds provided by (1) theDepartment of Defense, Office of Naval Research, under contract numbersN00014-93-0618 and N00014-93-1-1120, and (2) the National ScienceFoundation, Grant No. DMR-9705343. The federal government may haverights in this invention.

FIELD OF THE INVENTION

[0003] This invention concerns a method for forming organized arrays ofmetal, alloy, semiconductor and/or magnetic clusters for use in themanufacture of electronic devices, such as high density memory storageand nanoelectronic devices.

BACKGROUND OF THE INVENTION

[0004] Fundamentally new technologies are required to continueincreasing device integration density and speed. Conventionalmetal-oxide semiconductor-field-effect transistors soon will reachfundamental density and speed limits as a result of quantum mechanicaltunneling. In order to scale electronic device sizes down to nanometerdimensions, systems containing increasingly fewer numbers of particlesmust be considered.

[0005] The ultimate limit is a system in which the transfer of a singlecharge quanta corresponds to information transfer or some type oflogical operation. Such single electron systems are presently the focusof intense research activity. See, for example, Single Charge Tunneling,Coulomb Blockade Phenomena in Nanostructures, edited by H. Grabert andM. H. Devoret, NATO ASI Series B: Physics Vol. 294 (1992). These systemshave potential application to nanoelectronic circuits that haveintegration densities far exceeding those of present day semiconductortechnology. See, Quantum Transport in Ultrasmall Devices, edited by D.K. Ferry, H. L. Grubin, C. Jacoboni, and A. Jauho, NATO ASI Series B:Physics Vol. 342 (1995).

[0006] Single electron transistors based on the concept of Coulombblockade are one proposed technology for realizing ultra-dense circuits.K. K. Likharev's Single Electron Transistors: Electrostatic Analogs ofthe DC SQUIDS,” IEEE Trans. Magn. 23:1142 (1987); and IBM J. Res. Dev.32:144 (1988). Coulomb blockade is the suppression of single electrontunneling into metallic or semiconductor islands. In order to achieveCoulomb blockade, the charging energy of an island must greatly exceedthe thermal energy. To reduce quantum fluctuations the tunnelingresistance to the island should be greater than the resistance quantumh/e². Coulomb blockade itself may be the basis of conventional logicelements, such as inverters. Id.

[0007] Equally promising is the fact that the Coulomb blockade effectcan be used to pump charges one-by-one through a chain of dots torealize a frequency-controlled current source in which the current isexactly equal to I=ef, where f is the clocking frequency. See, L. J.Geerligs et al.'s Frequency-locked Turnstile Device for SingleElectrons, Phys. Rev. Lett., 64:2691 (1990); and H. Pothier et al.'sSingle-Electron Pump Based on Charging Effects, Europhys. Lett. 17:249(1992). Such turnstile devices are of fundamental interest as highlyaccurate current standards.

[0008] The clocking of charge through an array is also one model ofinformation storage. It is possible that computation may be based onswitching of currents rather than charge which, due to the extremeaccuracy of single electron current sources, may be more robust towardsunwanted fluctuations than single electron transistor-based circuits.

[0009] One of the most promising technologies for realizing terabytememories is founded on the principle of the Coulomb blockade. Yano etal. have demonstrated room temperature operation of single electrondevices based on silicon nanocrystals embedded in SiO₂. K. Yano et al.'sRoom-Temperature Single Electron Memory, IEEE Trans. Electron. Devices,41:1628 (1994); and K. Yano et al.'s Transport Characteristics ofPolycrystalline-Silicon Wire Influenced by Single Electron Charging atRoom Temperature, Appl Phys. Lett., 67:828 (1995). Recently, a fullyintegrated 8×8 memory array using this technology has been reported. K.Yano et al.'s Single-Electron-Memory Integrated Circuit for Giga-to-TeraBit Storage, IEEE International Solid State Circuits Conference, p.266-267 (1996).

[0010] Microelectronic devices based on the principle of Coulombblockade have been proposed as a new approach to realizing electroniccircuits or memory densities that go beyond the predicted scaling limitfor present day semiconductor technology. While the operation of Coulombblockade devices has been demonstrated, most operate only at greatlyreduced temperatures and require sophisticated nanofabricationprocedures. The size scales necessary for Coulomb blockade effects atsuch relatively elevated temperatures of about room temperature imposelimits on the number, uniformity and connectivity of quantum dots. As aresult, alternative methodologies of nanofabrication need to beinvestigated and developed.

SUMMARY OF THE INVENTION

[0011] The present invention provides a new process for making arrayscomprising metal, alloy, semiconductor and/or magnetic clusters. An“array” can be any arrangement of plural such clusters that is usefulfor forming electronic devices. Two primary examples of arrays are (1)electronic circuits, and (2) arrangements of computer memory elements,both of which can be in one or several planes.

[0012] “Clusters” as used herein refers to more than one, and typicallythree or more, metal, alloy, semiconductor or magnetic atoms coupled toone another by metal-type bonds. Clusters are intermediate in sizebetween single atoms and colloidal materials. Clusters made inaccordance with the present invention also are referred to herein as“nanoclusters.” This indicates that the radius of each such clusterpreferably is from about 0.7 to about 1.0 nm. A primary goal of thepresent invention is to provide electronic devices that operate at orabout room temperature. This is possible if the cluster size is madesmall enough to meet Coulomb blockade charging energy requirements atroom temperature. While cluster size itself is not dispositive ofwhether the clusters are useful for forming devices operable at or aboutroom temperature, cluster size is nonetheless quite important. Itcurrently is believed that clusters having radiuses much larger than themaximum value stated above likely will not be useful for formingelectronic devices that operate at or about room temperature.

[0013] The metal, alloy, semiconductor and/or magnetic clusters arebonded to “scaffolds” to organize the clusters into arrays. “Scaffolds”are any molecules that can be placed on a substrate in predeterminedpatterns, such as linear bridges between electrodes, and to whichclusters can be bonded to provide organized cluster arrays. Withoutlimitation, a preferred group of scaffolds comprise biomolecules, suchas polynucleotides, polypeptides, and mixtures thereof. Polypeptides arecurrently preferred molecules for forming scaffolds, and polypeptidescapable of forming a helices are particularly preferred scaffold-formingmolecules.

[0014] One embodiment of a method for forming arrays of metal, alloy,semiconductor and/or magnetic clusters first involves placing thescaffold on a substrate, most likely in a predetermined pattern. Arraysare formed by contacting the scaffold with plural, monodispersed(clusters of substantially the same size) ligand-stabilized metal,alloy, semiconductor and/or magnetic clusters. If the clusters are metalclusters, then the metal preferably is selected from the groupconsisting of Ag, Au, Pt, Pd and mixtures thereof. A currently preferredmetal is gold, and a currently preferred metal cluster is Au₅₅.

[0015] Clusters generally are bonded to the scaffold by ligand exchangereactions. Each cluster, prior to contacting the scaffold, includesplural exchangeable ligands bonded thereto. The ligand-exchangereactions involve exchanging functional groups of the scaffold for atleast one of the exchangeable ligands bonded to the cluster prior tocontacting the scaffold with the clusters. Examples of exchangeableligands suitable for forming metal clusters in accordance with theinvention can be selected from the group consisting of thiols,thioethers (i.e., sulfides), thioesters, disulfides, sulfur-containingheterocycles, 1°, 2° and perhaps 3° amines, pyridines, phosphines,carboxylates, nitriles, hydroxyl-bearing compounds, such as alcohols,and mixtures thereof. Thiols currently are preferred ligands forpracticing the present invention.

[0016] There are several methods for placing the scaffold ontosubstrates in predetermined patterns. For example, a first methodcomprises aligning scaffold molecules in an electrical field createdbetween electrodes on the substrate. It therefore will be appreciatedthat the scaffold molecules used must have sufficient dipoles to allowthem to align between the electrodes. This is one reason whypolypeptides that form α helices are preferred. The a helix imparts asufficient dipole to the polypeptide molecules to allow alignment of themolecules between the electrodes upon formation of an electrical field.One example of a polypeptide useful for forming scaffolds in accordancewith the present invention is polylysine.

[0017] A second method comprises polymerizing monomers, oligomers (10amino acids or nucleotides or less) or small polynucleotides orpolypeptides into longer molecules on the surface of a substrate. Forexample, scaffold molecules can be polymerized as a bridge betweenelectrodes on a substrate.

[0018] The present invention also provides compositions, one use forwhich is in the formation of metal and/or semiconductor arrays. Acurrently preferred embodiment of the composition comprisesmonodispersed, ligand-stabilized Au₅₅ metal clusters bonded to apolypeptide in the shape of or capable of forming an a helix with themetal clusters bonded thereto. The metal clusters have metal-clusterradiuses of from about 0.7 nm to about 1.8 nm, and preferably from about0.7 nm to about 1.0 nm.

[0019] An object of this invention is to provide methods for fabricatingone-, two-, and three-dimensional, scaffold-organized metal clusterarrays.

[0020] An object of this invention is to provide high density electronicor memory devices that operate on the principle of Coulomb blockade atambient temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram of an interdigitated electrode arrayhaving saw-tooth edges.

[0022]FIG. 2 is a schematic representation of a poly-L-lysine scaffoldhaving thiophenolate-stabilized nanoclusters coupled thereto.

[0023]FIG. 3 is a schematic representation of one method forincorporating gate electrodes at the molecular level.

[0024]FIG. 4 is a UV-vis spectra (methylene chloride solution) of goldclusters with ligands (a) ODT, (b) Pth, and (c) MBP, and where (d) isstarting material and (e) is a sample of larger ODT-stabilzed clusters.

[0025]FIG. 5 is a TEM of ODT-stabilized clusters (aerosol-deposited frommethylene chloride solution onto a carbon-coated copper grid).

[0026]FIG. 6 is an electron micrograph of a patterned gold clusterstructure.

[0027]FIG. 7 is a graph illustrating current-voltage (I-V)characteristics of Au₅₅[P(C₆H₅)₃]₁₂Cl₆ at 195K, 295K and 337K.

[0028]FIG. 8 is a graph illustrating observed current plateaus as afunction of the applied frequency at 195K, with the inset illustratingthe plateau at f=0.626 MHz.

[0029]FIG. 9 is a graph illustrating current versus reduced voltage at195K.

[0030]FIG. 10 is a graph illustrating current-voltage (I-V)characteristics of a poly-L-lysine scaffold decorated with11-mercaptoundeconic ligand-stabilized gold clusters.

[0031]FIG. 11 is a TEM image of a TEM grid having a poly-L-lysinescaffold decorated with 11-mercaptoundeconic ligand-stabilized goldclusters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The general steps used to produce organized arrays comprisingmetal, alloy, semiconductor and/or magnetic clusters in accordance withthe present invention include (1) attaching molecular scaffolds tosubstrates in predetermined patterns, (2) forming monodispersed,relatively small (i.e., nanocluster size) ligand-stabilized metal,alloy, semiconductor and/or magnetic clusters, (3) coupling theligand-stabilized clusters to the scaffolds to form organized arrays,(4) coupling electrical contacts to the organized arrays, and (5) usingsuch constructs to form electronic, particulary nanoelectronic, devices.The substrate generally is a metal, glass or semiconductor material.

[0033] Currently, most efforts have been directed to developing workingdevices using metal clusters. Certain of the following passagestherefore focus on describing how to make and use devices based on metalcluster arrays. It should be understood, however, that any reference inthis application to “metal clusters” or “clusters” typically also refersto alloy clusters, semiconductor clusters, magnetic clusters, andcombinations thereof.

[0034] Important features of the present invention include the smallphysical size of the metal clusters, the ligand exchange chemistry andthe nature of the ligand shell produced by the ligand exchangechemistry. The small physical size of the metal clusters provides alarge Coulomb charging energy. The ligand-exchange chemistry provides ameans to taylor the ligand shell for a particular purpose and immobilizethe clusters on biomolecules. And, the ligand shell offers a uniform andchemically adjustable tunnel barrier between cluster cores.

[0035] The following paragraphs describe the present invention ingreater detail.

I. Forming Monodispersed Ligand-Stabilized Clusters

[0036] A feature of the present invention is the recognition thatmonodispersed, relatively small metal clusters can be used to developelectronic devices that operate at or about room temperature based onthe Coulomb blockade effect. “Monodispersed” refers to the formation ofa population of metal clusters of substantially the same size, i.e.,having substantially the same radiuses (or diameters). In contrast,prior-art approaches typically have used polydispersed metal clusterswhere the size of the metal clusters is not substantially uniform. Acompletely monodispersed population is one in which the size of themetal clusters is identical. However, complete monodispersity isdifficult, if not impossible, to achieve. And complete monodispersity isnot required to produce devices operating at room temperature based onthe Coulomb blockade effect. Nevertheless, as the dispersity of thecluster population proceeds from absolute monodispersity towardspolydispersity the likelihood that the device will operate reliably atroom temperature based on the Coulomb blockade effect decreases.

[0037] Moreover, as the radius of the metal cluster decreases, theintrinsic capacitance gets smaller. As capacitance gets smaller, thecharging energy of the cluster gets larger. Coulomb blockade effects areobserved when the charging energy exceeds the thermal energy at roomtemperature. Prior approaches have used clusters having radiusesgenerally larger than would be useful for forming devices that operateat room temperature based on the Coulomb blockade effect. In contrast,the present invention forms metal “nanoclusters” having relatively smallradiuses. The size requirement for clusters made in accordance with thepresent invention can be established in at least two ways, (1) bystating absolute radius lengths, and (2) by comparing the radius of thecluster in question to the radius of gold clusters made having magicnumbers (see the discussion provided below) of gold atoms.

[0038] In terms of absolute numbers, “nanocluster” is defined herein asa cluster having a radius of from about 0.7 nm to about 1.8 nm (7 Å toabout 18 Å), preferably from about 0.7 nm to about 1.25 nm (7 Å to about12.5 Å), and even more preferably from about 0.7 nm to less than orequal to 1.0 nm (7 Å to less than or equal to 10 Å). These radiuslengths refer solely to the radius of the metal cluster, and not theradius of the metal cluster and ligand sphere.

[0039] With its insulating ligand shell, the diameter of theligand-stabilized metal cluster can vary. The size of the ligand shellmay influence the electron tunneling rate between clusters. Tunnelingrate is exponentially related to the thickness of the ligand shell. As aresult, the diameter of the ligand shell may be tailored for aparticular purpose. It currently is believed that the diameters forligand-stabilized clusters useful for practicing the present inventionshould be from about 2.5 nm to about 5 nm. The relatively large metalclusters made previously do not provide a large Coulomb charging energyand do not operate at room temperature, and instead generally onlyoperate at temperatures of from about 50 mK to about 10K.

[0040] “Bare” clusters, i.e., those without ligand shells, also may beuseful for practicing the present invention. For example, bare clusterscan be used to form electrical contacts.

[0041] Still another consideration is the distance between the edges ofmetal cluster cores. It currently is believed that the maximum distancebetween the edges of cluster cores for clusters useful for practicingthe present invention is about 5 mm (50 Å), and ideally is on the orderof from about 1 to about 2 nm (10-20 Å).

[0042] Originally it was believed that clusters in accordance with thepresent invention generally should include numbers of atoms that arebased on the so-called “geometric magic numbers” of atoms surrounded bya ligand shell. Geometric magic numbers result from the most denselypacked arrangement of atoms that form a “sphere.” Magic numbers aregiven by Formula 1 below $\begin{matrix}{1 + {\sum\limits_{n = 1}^{k}{\left( {{10n^{2}} + 2} \right)1}}} & {{Formula}\quad 1}\end{matrix}$

[0043] where k is an integer that represents the number of shells ofmetal atoms surrounding a central atom. Noble metal clusters with k=2,4, 6, 7 and 8 have been synthesized and stabilized by a ligand shell.While clusters having magic numbers of atoms will work to practice thepresent invention, it has now been determined that magic numbers ofatoms likely are not required to provide clusters useful for practicingthe present invention.

[0044] Solely by way of example, the most likely metals to be used toform ligand-stabilized metal clusters in accordance with the presentinvention can be selected from the group consisting of silver (Ag), gold(Au), platinum (Pt), palladium (Pd), and mixtures thereof. “Mixturesthereof” refers to having more than one type of metal cluster coupled toa particular scaffold, or different metal clusters bonded to differentscaffolds used to form a particular electronic device. It also ispossible that metal alloy clusters, e.g., gold/palladium clusters, canbe used to form cluster arrays and electronic devices in accordance withthe present invention.

[0045] Gold is the currently preferred metal for formingligand-stabilized monodispersed metal clusters. This is because (1) theligand exchange chemistry for gold nanoclusters and the nature of theligand shell formed about gold is well understood, (2) Au₅₅ has adiameter of about 1.2 μm, which has proved ideal for forming organizedmetal arrays that exhibit the Coulomb effect at or about roomtemperature, and (3) it is possible to prepare nearly monodispersed goldclusters without lengthy purification requirements, such as lengthycrystallization processes.

[0046] Assuming that magic numbers do provide benefit, the magic numbersof gold, palladium and platinum atoms for use with the present inventionare 13, 39, 55, 147 and 309. 55 is the currently preferred magic number(represented as Au₅₅, Pd₅₅ and Pt₅₅). The magic number of silver atomsfor silver metal clusters useful for practicing the present inventionlikely are the same as for gold, but this has not yet been verified.

[0047] Semiconductor materials also likely are useful for practicing thepresent invention. Likely semiconductor materials that can be made intonanoclusters and stabilized with ligand spheres include, withoutlimitation, cadmium selenide, zinc selenide, cadmium sulfide, cadmiumtellurite, cadmium-mercury-tellurite, zinc tellurite, gallium arsenide,indium arsenide and lead sulfide.

[0048] Magnetic particles also can be used to decorate scaffolds inaccordance with the present invention. An example, without limitation,of such magnetic particles is iron oxide (Fe₂O₃).

II. Ligands A. Background

[0049] Once a suitable metal, alloy, semiconductor and/or magneticmaterial is selected for forming nanoclusters, ligands for bonding tothe clusters also must be selected. The assembly of clusters intoCoulomb blockade structures requires molecular-scale organization of theclusters while simultaneously maintaining the insulating ligand spherebetween individual clusters. The clusters also must be coupled to thescaffold in a sufficiently robust manner to allow for fabrication ofdevices incorporating cluster arrays. This can be accomplished by ligandexchange reactions. The selection of ligands for forming an insulatingligand layer about the cluster and for undergoing ligand exchangereactions therefore is an important consideration. A list of criteriauseful for selecting appropriate ligands includes, but may not belimited to, (1) the ligands should be capable of undergoing reactionswith the scaffold, such as ligand-exchange, acid-base or intercelationreactions (2) the ligands preferably increase the solubility of theligand-metal cluster complexes in organic solvents, which helpssynthesize metal clusters and perform subsequent reactions, and (3) theligands selected preferably form well ordered metal-ligand complexeshaving diameters as stated above.

B. Classes of Ligands

[0050] Ligands deemed most suitable for forming metal clusters inaccordance with the present invention can be selected, withoutlimitation, from the group consisting of: thiols (RSH); thioethers (alsoknown as sulfides, R—S—R′); thioesters (R—S₂H); disulfides (R—S—S—R′);sulfur-containing heterocycles, such as thiophene; 1□, 2□ and perhaps 3□amines (RNH₂, R₂NH and R₃N, respectively), particularly 1□ amines;pyridines; phosphines (R₃P); carboxylates (RCO₂ ⁻); nitriles (RCN);hydroxyl-bearing compounds, such as alcohols (ROH); and mixturesthereof. Additional guidance concerning the selection of ligands can beobtained from Michael Natan et al.'s Preparation and Characterization ofAu Colloid Monolayers, Anal. Chem., 67:735-743 (1995), which isincorporated herein by reference.

[0051] Organic sulfur-containing molecules (e.g., thiols, thioethers,thioesters, disulfides, sulfur-containing heterocycles, and mixturesthereof) currently are the preferred class of ligands. Thiols are thecurrently preferred type of sulfur-containing ligand for severalreasons. For example, thiols have an affinity for gold, which often isformed into electrodes or electrode patterns. Moreover, thiols have beenshown to be good ligands for stabilizing gold clusters. And, manythiol-based ligands are commercially available. The thiols formligand-stabilized metal clusters having a formula M_(x)(SR)_(n) whereinM is a metal, R is an alkyl chain or aromatic group, x is a number ofmetal atoms that provide metal clusters having the characteristicsdescribed above, and n is the number of thiol ligands attached to theligand-stabilized metal clusters.

C. Organic Portion of Ligands

[0052] The organic portion of ligands useful for practicing the presentinvention also can vary. For example, the length of the alkyl chain canbe varied to obtain particular features desired in the ligand-stabilizedmetal clusters. These include the solubility of the metal clusters insolvents used to carry out the present invention and the size andinsulating characteristics of the ligand-stabilized metal clusters.Currently, alkyl chains having from about 2 carbon atoms to about 20carbon atoms are deemed most suitable for practicing the presentinvention.

[0053] Aryl-type ligands, i.e., aromatic groups such as phenyl rings,containing or having sulfur atoms coupled thereto also have been used asligands for forming ligand-stabilized metal clusters. For example,mercaptobiphenyl (HS-phenyl-phenyl) has been used to formligand-stabilized gold clusters. The aromatic rings of such compoundslikely will be functionalized to include functional groups capable ofreacting with the scaffold molecules. For example, the aromatic ringsmight include acidic groups, such as carboxylic acids, for acid-basereactions with functional groups of the scaffold molecules, such asamines.

[0054] Aromatic ligands are quite useful for producing rigid arrays,which helps stabilize the electron transport properties. For thisreason, aryl ligands currently are considered preferred ligands forpracticing the present invention. But, small alkyl groups, such asthioproprionic acid, also provide rigid ligand systems.

[0055] Ligands that intercalate into DNA also can be used. This allows ameans for attaching the metal clusters to DNA molecules. Typically, theDNA intercalating ligands include rigid π systems. Examples of such DNAintercalating ligands include, without limitation, anthraquinone andphenanthridinium derivatives. The DNA intercalating ligands also can bemade DNA-sequence dependent. Thus, DNA having particular sequences canbe used as a scaffold that is intercelated at predetermined portions ofthe scaffold. This provides a method for designing the spacing betweenmetal clusters. The intercalating ligands also can be photocrosslinkedto provide a more rigid system.

D. General Method for Producing Ligand-Stabilized Metal Clusters

[0056] The general approach to making ligand-stabilized metal clustersfirst comprises forming monodispersed metal clusters having displaceableligands. This can be accomplished by directly forming monodispersedmetal clusters having the appropriate ligands attached thereto, but ismore likely accomplished by first forming monodispersed,ligand-stabilized metal clusters which act as precursors for subsequentligand-exchange reactions with ligands deemed more useful for practicingthe present invention.

[0057] One example, without limitation, of a monodispersed gold clusterthat has been produced and which is useful for subsequentligand-exchange reactions with the ligands listed above isAu₅₅[P(C₆H₅)₃]₁₂Cl₆. A procedure for making monodispersedAu₅₅[P(C₆H₅)₃]₁₂Cl₆ nanoclusters is provided by G. Schmid'sHexachlorodecakis(triphenylphosphine)-pentapentacontagold, Au ₅₅ [P(C ₆H ₅)₃]₁₂ Cl ₆ , Inorg. Syn., 27:214-218 (1990). Schmid's publication isincorporated herein by reference. Schmid□s synthesis involves thereduction of AuCl[Ph₃]₆. Example 1 below also discusses the synthesis ofAu₅₅[P(C₆H₅)₃]₁₂Cl₆. One advantage or Schmid□s synthesis is therelatively small size distribution of clusters produced by the method,e.g., 1.4±0.4 nm.

[0058] Once ligand-stabilized monodispersed metal clusters are obtained,such clusters can be used for subsequent ligand-exchange reactions, aslong as the ligand-exchange reaction is readily facile and producesmonodispersed metal clusters. Prior to the present invention it was notappreciated that the Au₅₅[P(C₆H₅)₃]₁₂Cl₆ clusters could be used to formnearly monodispersed derivatives by ligand-exchange chemistry. In fact,some literature reports indicated that it was difficult, if notimpossible, to form linked metal clusters by ligand-exchange reactions.See, for example, Andres et al.'s Self-Assembly of a Two-DimensionalSupperlattice of Molecularly Linked Metal Clusters, Science,273:1690-1693 (1996).

[0059] To perform ligand-exchange reactions, a reaction mixture isformed comprising the metal cluster having exchangeable ligands attachedthereto and the ligands to be attached to the metal cluster, such asthiols. A precipitate generally forms upon solvent removal, and thisprecipitate is then isolated by conventional techniques. See, Examples 2and 3 for further details concerning the synthesis of ligand-stabilizedmetals.

III. Molecular Scaffolds A. Background

[0060] Metal clusters produced as stated above are coupled to molecularscaffolds. “Coupling” as used herein refers to some interaction betweenthe scaffold and the ligand-stabilized metal clusters such that themetal clusters become associated with the scaffold. Associated may meancovalently bound, but also can include other molecular associations,such as electrostatic interactions. “Coupling” most typically refers toattaching clusters to the scaffolds by either (1) ligand exchangereactions where functional groups of the scaffold molecules, such assulfur-containing functional groups or amines, exchange with ligandsforming the metal-ligand complex, (2) acid-base type reactions betweenthe ligands and molecules of the scaffold, or (3) intercelation of aligand into a DNA helix.

B. Scaffolds Comprising Biomolecules

[0061] To form useful devices, the scaffolds must be disposed on asubstrate in predetermined patterns to which electric contacts can bemade. The scaffolds of the present invention can comprise biomolecules,such as polynucleotides, polypeptides and mixtures thereof, and henceare most appropriately referred to as biomolecular scaffolds. There issome precedent for using polynucleotides for forming molecularscaffolds. See, for example, C. A. Mirkin et al.'s A DNA-Based Methodfor Rationally Assembling Nanoparticles into Macroscopic Materials,Nature, 382:607 (1996); and A. P. Alivisatos et al.'s Organization of‘Nanocrystal Molecules’ using DNA, Nature, 382:609 (1996). Each of thesereferences is incorporated herein by reference. Polynucleotides providea different spacing between metal clusters than do polypeptides. Thus,spacing between metal clusters can be varied by changing the nature ofthe scaffold. Polypeptides may provide the best spacing for theformation of electronic devices operating at room temperature based onthe Coulomb blockade effect.

[0062] Preferred polypeptides are those polypeptides that form α helicalsecondary structures. Certain peptides, although attractive candidatesfrom the standpoint of being stabilizing ligands for the metal clusters,do not form a helices. However, many polypeptides do form a helices, andhence are good candidates for forming scaffolds in accordance with thepresent invention.

[0063] It also should be appreciated that the polypeptide can be a“homopolypeptide,” defined herein to refer to polypeptides having onlyone type of amino acid. One example of a homopolypeptide ispoly-L-lysine. The free base form of polylysine readily forms an αhelix. Moreover, lysine provides a terminal amino group that is orientedfavorably in the α helix for ligand exchange reactions with theligand-stabilized metal clusters. Homopolypeptides generally have beenused in the practice of the present invention for several reasons.First, certain homopolypeptides are commercially available, such aspolylysine. Second, homopolypeptides provide more predictable α helixformation with the side chains oriented outwardly from the α helix atknown, predictable distances. This allows the polypeptide to be designedfor a particular purpose.

[0064] The peptide also may be a “heteropolypeptide” (having two or moreamino acids), or block copolymer-type polypeptides (formed from pluraldifferent amino acids with identical amino acids being organized inblocks in the amino acid sequence), as long as such peptides (1) form αhelices, and (2) provide functional groups positioned and capable ofengaging in ligand exchange reactions with the monodispersed metalclusters.

[0065] Most amino acids can be used to form suitable homo- orheteropolypeptides. Examples of particularly suitable amino acidsinclude, but are not limited to, naturally occurring amino acids such asarginine, tyrosine, and methionine; and nonnaturally occurring aminoacids such as homolysine and homocysteine.

IV. Placing Scaffolds on Substrates A. General Discussion

[0066] The scaffold simply may be placed on the surface of thesubstrate, in contrast to more tightly adhering the polypeptide to thesubstrate such as through electrostatic or covalent bonds. As usedherein, the term “substrate” refers to any material, or combination ofmaterials, that might be used to form electronic devices. For example,the substrate might be selected from the group consisting of silicon,silicon nitride, ultraflat glass, metals, and combinations thereof.

[0067] Simply placing the scaffold on the surface without consideringwhether to electrostatically or covalently bind the scaffold to thesubstrate simplifies the process for making working devices. Placing thescaffold on the surface of the substrate can be accomplished by (1)forming solutions containing the molecular scaffold, (2) placing thesolution containing the scaffold onto a substrate, such as by spincoating the solution onto a substrate, and (3) allowing the solvent toevaporate, thereby depositing the solid molecular scaffold onto thesubstrate surface.

[0068] If simple deposition of the scaffold onto the substrate does notproduce a sufficiently robust device, then the scaffold might be moretightly coupled to the substrate. One method for accomplishing this isto use compounds that act as adhesives or tethers between the substrateand the molecular scaffold. Which compounds to use as adhesives ortethers depends on the nature of the substrate and the metal cluster.For example, amino-silane reagents may be used to attach molecularscaffolds to the substrate. The silane functional group allows thetether to be coupled to a silicon, glass or gold substrate. Thisprovides a tether having a terminal amino group that can be used toreact with the scaffold to tether the scaffold to the substrate. Theterminal amino group also can be used as an initiation site for the insitu polymerization of polypeptides using activated amino acids. Anotherclass of tethers particularly useful for attaching polylysine tosubstrates is the ω-carboxyalkanethiols (⁻O₂C—R—SH).

B. Organization of Scaffolds on Substrates

[0069] There are at least four methods for forming organized moleculararrays, particularly linear arrays, on the surface of substrates. Thefirst comprises depositing dilute solutions of scaffold molecules ontosubstrates. The second comprises aligning α-helical polypeptides betweenelectrodes. The third comprises growing polypeptide chains between twoor more electrodes beginning from an initiation site placed on anelectrode. And the fourth comprises forming DNA scaffolds betweenelectrodes. Each of these approaches is discussed below.

1. Deposition from Dilute Solutions

[0070] First, isolated molecular scaffolds can be prepared by depositinghighly dilute solutions onto substrate surfaces. Alternatively, this canbe accomplished by dilution of the molecular scaffold film with aninert, α-helix polypeptide such as poly-γ-benzyl-L-glutamate. See,Poly(γ-Benzyl-L-Glutamate) and Other Glutamic Acid Containing Polymers,H. Block (Gordon & Breach, NY) 1983.

2. Aligning Polypeptides in a High Electrical Field

[0071] The second, and likely most practical method, for providing ascaffold on a substrate is to align a polypeptide “bridge” in anelectrical field produced between two electrodes. FIG. 1 illustrates sawtooth electrodes 10 comprising electrodes 12-20 that are placed on asubstrate by conventional methods, such as electron-beam lithography,thermal evaporation, or lift-off techniques. A solution comprising thescaffold molecules is first formed and then applied to the surface ofthe substrate having the electrode pattern placed thereon, such as asubstrate having the electrode pattern of FIG. 1. α-Helical polypeptidesself-align (pole) in the presence of an applied magnetic field orelectrical field (typically 20 Vcm⁻¹). See, S. Itou's Reorientation ofPoly-γ-benzyl-L-glutamate Liquid Crystals in an Electric Field, Jpn.Appl. Phys., 24:1234 (1985). Presumably this is due to their largediamagnetic anisotropy. See, C. T. O'Konski et al.'s Electric Propertiesof Macromolecules IV. Determination of Electric and Optical ParametersFrom Saturation of Electric Birefringence in Solutions, J. Phys. Chem.,63:1558 (1959). An electric field is generated between the electrodes,such as the points of the saw tooth illustrated in FIG. 1. This localfield between the two points causes the scaffold to align between thepoints. The solvent is evaporated to provide scaffolds oriented betweenthe electrodes.

[0072] Based on the above, it will be apparent that the dipole moment ofthe scaffold influences whether the polypeptide can be oriented betweenthe two electrodes, and the efficiency of the orientation. This is onereason why a helical polypeptides are a currently preferred polypeptidesfor forming scaffolds. The hydrogen bonds formed in the α helix allorient in the same direction, thereby imparting a dipole to thesecondary α helical structure. It currently is believed that the dipoleis primarily the result of the α helix, and not the side chains. As aresult, preferred polypeptides for practicing the present invention arethose that form α helices.

3. Growing Polypeptides Between Electrodes

[0073] In some instances, it may be desirable to use scaffolds to bridgedirectly between two electrical contacts of interest. This can beaccomplished by first placing initiating sites on the electrodes, andthen “growing” polypeptides between the initiation sites on theelectrodes to form a bridge. One example of how this would beaccomplished is to attach a tether to an electrode, the tether having apendant functional group that is capable of forming peptide bonds whenreacted with an activated amino acid. The most likely pendant functionalgroup for this purpose is a 1° amine.

[0074] To provide a specific example to illustrate the procedure, atether comprising an alkyl chain having both a terminal amino group anda terminal sulffiydryl group (i.e., an amino-thiol, HS—R—NH₂) is reactedwith a gold electrode using conventional chemistry. This covalentlyattaches the sulfhydryl group of the tether to the metal (i.e.,Au-S—R—NH₂). The terminal amino group is then used to initiatepolymerization of a polypeptide using activated amino acids, perhaps inthe presence of an applied field, between the two electrodes. Thepolymerization is accomplished by supplying activated amino acids forreaction with the primary amine in a chain-growing reaction whichserially couples amino acids to the end of the growing chain andregenerates the primary amine for subsequent reaction with anotheractivated amino acid.

[0075] Activated amino acids are commercially available and aredescribed in the literature. One example, without limitation, of anactivated amino acid for formation of peptide bonds in this manner isN-carboxyanhydride (NCA) amino acids. NCA amino acids react withsurface-bound initiator sites (e.g., the primary amino groups) to begina ring-opening polymerization of the NCA-amino acid. See, J. K.Whitesell et al.'s Directionally Aligned Helical Peptides on Surfaces,Science, 261:73 (1993). Whitesell's publication is incorporated hereinby reference.

[0076] When NCA polymerization is performed under the influence of anelectric field applied between two electrodes it is possible to “grow”the polypeptide scaffolds from one electrode to the other. One specificexample of an NCA amino acid that can be used for this purpose is thatderived from N_(ε)-benzyloxycarbonyl-L-lysine. The amino acid sidechains of this compound can be deprotected using trimethylsilyl iodide.Deprotection yields the poly-L-lysine scaffold.

[0077] Working embodiments of the present invention generally have usedpolylysine as the polypeptide useful for forming the molecular scaffold.Polylysine was chosen because it includes a hydrocarbon chain thatextends the amino functional group, which can undergoligand-displacement reactions with the ligand-stabilized metal cluster,out and away from the polypeptide backbone. Thus, two important criteriafor selecting polypeptides for use as molecular scaffolds are (1) doesthe polypeptide form a helices, and (2) do the amino acid side chainsprovide functional groups that are metal-cluster stabilizing and capableof undergoing ligand-exchange reactions with the ligand-stabilized metalclusters.

4. Forming Polynucleotide Scaffolds

[0078] Methods for providing polynucleotide scaffolds also recently havebeen discovered. See, for example, (1) E. Braun et al., “DNA TemplatedAssembly and Electrode Attachment of a Conducting Silver Wire,” Nature,p. 775 (1998); (2) N. Seeman, “DNA Components for MolecularArchitecture,” Accounts of Chemical Research, 30:357 (1997); Qi J., etal. “Ligation of Triangles Built from Bulged 3-Arm DNA BranchedJunctions,” J. Am. Chem. Soc., 118:6121 (1996); and C. Niemeyer et al.,“DNA as a Material for Nanotechnology,” Angewandte Chemie, InternationalEdition in English, 36:585 (1997). Each of these references isincorporated herein by reference. The Braun reference provides a methodfor positioning a DNA molecule between electrodes spaced by a particulardistance, such as about 10 μm. Double stranded DNA, with single strandedsticky ends, and a pair of electrodes that have single stranded DNAattached thereto that is complementary to the sequence of the stickyends of the DNA are prepared. Annealing the sticky ends to thesingle-stranded primers allows coupling of double stranded DNA betweentwo electrodes spaced by a known distance.

V. Decorating Scaffolds with Metal Clusters

[0079] To provide working electronic devices, clusters must be coupledto the scaffolds. FIG. 2 provides a schematic representation of apoly-L-lysine that is “decorated” with metal clusters, i.e., theclusters are coupled to the scaffold. A first consideration is whetherto decorate the scaffold with clusters prior to or subsequent to placingthe scaffold onto a substrate. Although both of these approaches work,there are some disadvantages with decorating the scaffold with theclusters prior to placing the scaffold on the substrate. This approachplaces clusters on all surfaces of the polypeptide, even those that comeinto contact with the underlying substrate. This is undesirable forseveral reasons. For example, such placement of the clusters mightinterfere with fixing the decorated scaffold to the substrate. And, itplaces clusters in locations in which they are not needed, and henceuses more valuable monodispersed clusters than needed.

[0080] Based on the above, a method which first places the scaffoldsonto a substrate, and subsequently decorates the scaffold with clustersis a currently preferred approach. This can be accomplished by firstforming a solution comprising the ligand-stabilized monodispersedclusters using a solvent that does not dissolve the scaffold. Candidatesolvents for this purpose include, without limitation, dichloromethaneand hexanes. The ligand-stabilized clusters are then introduced onto thescaffold and allowed to undergo reactions with the scaffold molecules,such as ligand-exchange or acid-base type reactions, thereby couplingthe ligand-stabilized clusters to the scaffold. See, Example 4 forfurther details concerning decorating scaffolds with clusters.

[0081] The present approach to producing decorated scaffolds also allowsfor good lateral definition, which is a key feature of the presentinvention. “Lateral definition” refers to the width of an array. Priorto the present invention, the state of technology was capable ofproducing lines having a width of about 300 Å. With the presentinvention, lateral resolution is much improved, and is on the order ofabout 10 Å. In addition, branched polypeptides offer the possibility ofintroducing control electrodes and interconnects at the molecular level.

VI. Ultrafast, Ultrahigh Density Switching Devices

[0082] This section discusses the steps required to use the decoratedmolecular scaffolds of the present invention to produce ultrafast,ultrahigh density switching devices. First, a substrate is selected andcleaned. One example of a substrate is a silicon nitride chip or wafer.On top of this substrate would be placed electrical contacts. This couldbe accomplished using known technologies, such as lithography or thermalevaporation of a metal, such as gold.

[0083] Once a substrate is obtained having the electrical contactsplaced thereon, a scaffold is then placed on the surface using thetechniques described above. Thereafter, the substrate with scaffold istreated with monodispersed, ligand-stabilized clusters to attach suchclusters to the scaffold. The organization of scaffold likely determinesthe particular device being made.

[0084] For a switching device, analogous to a transistor, saw toothelectrical contacts, such as those shown in FIG. 1, are deposited onto asubstrate and a scaffold then oriented therebetween. This provides twoarms of a transistor. A capacitance contact required to provide thethird arm of a transistor is imbedded in the substrate underneath themolecular scaffold. Direct electrical contact with this “gate” imbeddedin the substrate is not actually required.

[0085] Alternatively, a third contact arm could be incorporated into thetemplate. FIG. 3 is a schematic representation of a scaffold useful forthis purpose. For example, a polypeptide of a particular length, e.g., a25-mer or 50-mer, could first be coupled to an electrode. A branchingportion of the scaffold could then be attached, thereby forming anelectrical arm, or plural such arms, for further providing single ormultiple gate electrodes to the template. The scaffold is then coupledbetween two electrodes subsequent to the formation of this contact arm,or arms.

[0086] The method of the present invention can be used to form a varietyof standard circuit components to implement Boolean logic functions.These circuit components include, but are not limited to, AND, NAND,NOR, OR and Exclusive OR gates. Additionally, multiplexers andmuliplexer-based circuits can be created and used to implement Booleanlogic functions.

VII. EXAMPLES

[0087] The following examples are provided to illustrate certainparticular features of the present invention. These examples should notbe construed to limit the invention to the particular featuresdescribed.

Example 1

[0088] This example describes the syntheses of Au₅₅(PPh₃)₁₂Cl₆.Au[P(C₆H₅)₃]Cl was obtained from Aldrich Chemical Company. This compoundwas reduced using diborane (B₂H₆), which was produced in situ by thereaction of sodium borohydride (NaBH₄) and borontriflouride etherate[BF₃.O(C₂H₅)]. Au[P(C₆H₅)₃]Cl was combined with diborane in benzene toform Au₅₅(PPh₃)₁₂Cl₆. Au₅₅(PPh₃)₁₂Cl₆ was purified by dissolution inmethylene chloride followed by filtration through Celite. Pentane wasthen added to the solution to precipitate a black solid. The mixture wasfiltered and the solid was dried under reduced pressure to provideAu₅₅(PPh₃)₁₂Cl₆ in approximately 30% yield.

Example 2

[0089] This example describes the synthesis of Au₅₅(SC₁₈H₃₇)₂₆.Dichloromethane ({tilde over (≈)}10 ml), Au₅₅(PPh₃)₁₂Cl₆ (20.9 mg) andoctadecylthiol (23.0 mg) were combined in a 25 ml round bottom. A blacksolution was produced, and this solution was stirred under nitrogen atroom temperature for 36 hours. The solvent was then removed underreduced pressure and replaced with acetone. This resulted in theformation of a black powder suspension. The solid was then isolated byvacuum filtration and washed with acetone (10×5 ml). After the finalwash, the solid was redissolved in hot benzene. The benzene was removedunder reduced pressure with gentle heating to yield a dark brown solid.

[0090] The solid material was then subjected to UV-VIS (CH₂Cl₂, 230-800nm), ¹HNMR (133 MHz), ¹³CNMR, X-ray photoelectron spectroscopy (XPS) andatomic force spectroscopy. These analytical tools were used tocharacterize the structure of the compound produced, and such analysisindicated that the structure of the metal-ligand complex wasAu₅₅(SC₁₈H₃₇)₂₆.

[0091] X-ray photoelectron spectroscopy (XPS) data also was collectedconcerning Au₅₅(SC₁₈H₃₇)₂₆. This involved irradiating molecules withhigh-energy photons of fixed energy. When the energy of the photons isgreater than the ionization potential of an electron, the compound mayeject the electron, and the kinetic energy of the electron is equal tothe difference between the energy of the photons and the ionizationpotential. The photoelectron spectrum has sharp peaks at energiesusually associated with ionization of electrons from particularorbitals. X-ray radiation generally is used to eject core electrons frommaterials being analyzed. Clifford E. Dykstra's Quantum Chemistry &Molecular Spectroscopy, pp. 296-295 (Prentice Hall, 1992).Quantification of the data provided by XPS analysis of Au₅₅(SC₁₈H₃₇)₂₆made according to this example showed that Au 4f comprised about 67.38%and S 2p constituted about 28.01%+4.17%, which suggests a formula ofAu₅₅(SC₁₈H₃₇)₂₆.

[0092] Quantification of XPS spectra gave a gold-to-sulfur ratio ofabout 2.3:1.0 and shows a complete absence of phosphorus or chlorine. Aswith Au₅₅(PPh₃)₁₂Cl₆, a broad doublet is observed for the Au 4f level.The binding energy of the Au 4f 7/2 level is about 84.0-84.2 eV versusthat of adventitious carbon, 284.8 eV. This indicates absence of Au(I)and is similar to binding energies obtained for clusters such asAu₅₅(PPh₃)₁₂Cl₆. The binding energy of the S 2p 3/2 peak ranges from162.4 to 162.6 eV for the series of clusters. These values are shiftedto lower energy than those found for free thiols (163.3-163.9 eV) andare close to the values reported for thiolates bound to gold(162.0-162.4 eV). The possibility that unattached thiols may be presentin the sample is unambiguously ruled out by ¹H and ¹³C NMR.

[0093] Thermal gravimetric analysis confirms the Au:S ratio obtainedfrom XPS. On heating to 600° C., ODT-stabilized clusters display a 40%mass loss, corresponding to 26 ODT ligands on an assumed 55-atom goldcluster. This ratio alludes to the retention of a small cluster size. Asample of the larger hexadecanethiol-stabilized gold cluster has beenshown to give a 33.5% mass loss, corresponding to from about 95 to about126 ligands per cluster (diameter=2.4 nm).

[0094] Optical spectra of gold colloids and clusters exhibit asize-dependent surface plasmon resonance band at about 520 nm (See. FIG.4). In absorption spectra of ligand-exchanged clusters produced asstated in this example, the interband transition typically observed forsmall clusters including Au₅₅(PPh₃)₁₂Cl₆ was observed. Little or noplasmon resonance was observed, consistent with a cluster size of about1.7 nm or less. For the ODT-passivated cluster, no plasmon resonance wasobserved.

[0095] Quantitative size information can be obtained using transmissionelectron microscopy (TEM). The core size obtained from TEM images of theODT-stabilized cluster (FIG. 5) is found to be 1.7±0.5 nm and is in goodagreement with that obtained from atomic force microscope images.

[0096] Atomic force microscopy (AFM) also was performed on theAu₅₅(SC₁₈H₃₇)₂₆ produced according to this example. The analysisproduced a topographical representation of the metal complex. AFM probesthe surface of a sample with a sharp tip located at the free end of acantilever. Forces between the tip and the sample surface cause thecantilever to bend or deflect. The measured cantilever deflections allowa computer to generate a map of surface topography. Rebecca Howland etal.'s A Practical Guide to Scanning Probe Microscopy, p. 5, (ParkScientific Instruments, 1993). The AFM data showed heights of 1.5 m forsingle clusters and aggregates subjected to high force. This correspondsto the size of the gold core clusters. This helped establish that thegold clusters of this example were close to the correct size for formingdevices in accordance with the present invention.

[0097] In a manner similar to that described above for Example 2, thiolstabilized structures also have been made using 1-propanethiol.

Example 3

[0098] This example describes the synthesis of Au₅₅(SPh—Ph)_(x).Dichloromethane {tilde over (˜)}(≈10 ml), Au(PPh₃)₁₂Cl₆ (25.2 mg) and4-mercaptobiphenyl (9.60 mg) were combined in a 25 ml round bottom. Ablack solution was produced, and this solution was stirred undernitrogen at room temperature for 36 hours. The solvent was removed underreduced pressure and replaced with acetone. This resulted in theformation of a black powder suspension. The solid was isolated by vacuumfiltration and washed with acetone (6×5 ml). The solvent was thenremoved under reduced pressure to yield 16.8 mg of a dark brown solid.

[0099] The solid material was subjected to UV-VIS (CH₂Cl₂, 230-800 nm),¹HNMR (133 MHz), ¹³CNMR, X-ray photoelectron spectroscopy (XPS) andatomic force spectroscopy as in Example 2. This data confirmed thestructure and purity of the metal complex, and further showed completeligand exchange. For example, quantification of the XPS data madeaccording to this example showed that Au 4f comprised about 71.02% and S2p constituted about 28.98%, which suggests a formula ofAu₅₅(S-biphenyl)_(25.)

[0100] AFM analysis showed isolated metal clusters having measuringabout 2.5 nm which correlates to the expected size of the gold core witha slightly extended sphere.

[0101] Thiol-stabilized clusters as produced above display remarkablestability relative to Au₅₅(PPh₃)₁₂Cl₆, which undergoes decomposition insolution at room temperature to give bulk gold and AuCl[PPh₃]. Nodecomposition for the thiol-stabilized clusters was observed, despitethe fact that some samples were deliberately stored in solution forweeks. In other tests, the mercaptobiphenyl andoctadecylthiol-stabilized clusters (in the absence of free thiol) wereheated to 75° C. for periods of more than 9 hours in dilute1,2-dichloroethane solution with no resultant degradation. Underidentical conditions, Au₅₅(PPh₃)₁₂C1₆ is observed to decompose to Au(O)and AuCl[PPh₃] within 2 hours.

Example 4

[0102] This example demonstrates that polypeptide molecular templatescan be used to organize small, monodisperse nanoclusters into lineararrays of molecular dimension. A 1×10⁻⁸ M solution of poly-L-lysine ismixed with a large excess (1,000-fold) of “normal” gold 55 in methanolsolution. The amino sidechains of the poly-L-lysine replace some of thelabile triphenylphosphine ligands on the gold cluster and thus bind thecluster to the template. The decorated clusters precipitate out ofsolution onto a TEM grid. Single gold clusters that becomenon-specifically adsorbed on the grid will be removed by rinsing withbenzene. Transmission electron micrography (TEM) analysis will show thegold cores of the cluster and will indicate the extent to which thecluster have aggregated into low-dimensional arrays due totemplate-induced organization.

Example 5

[0103] This example describes the electron transfer properties oforganometallic structures formed by electron-beam irradiation ofAu₅₅[P(C₆H₅)₃]₁₂Cl₆. This compound was produced as stated above inExample 1. A solution of the gold cluster was made by dissolving 22 mgof the solid in 0.25 mL of CH₂Cl₂ and 0.25 mL of CH₂ClCH₂Cl. Asupernatant solution was spin coated onto a Si₃N₄ coated Si wafer at1500 rpm for 25 seconds immediately after preparation. The film waspatterned by exposure to a 40 kV electron beam at a line dosage of 100nC/cm. The areas of the film exposed to the electron beam adhered to thesurface and a CH₂Cl₂ rinse removed the excess film. This procedureproduced well defined structures. See, FIG. 6. These structures appearedto be smooth and continuous under SEM inspection. Attempts were made topattern the material using 254 nm UV lithography, but it was found to beinsensitive to this wavelength. The defined structures had dimensions assmall as 0.1 μm and AFM inspection measured the film thickness to be 50nm.

[0104] The organometallic samples were spin-coated with PMMA which waselectron-beam exposed and developed to define contact regions. Contactswere fabricated using thermal evaporation of 100 nm of gold andconventional liftoff procedures.

[0105] DC current-voltage (I-V) measurements of several samples weretaken. A shielded chamber, submerged in an oil bath, contained thesample mounted on a clean teflon stage. Rigid triaxial connections wereused to connect the sample to a constant DC voltage source andelectrometer. The oil bath temperature was controlled from 195 to 350K.Thermal equilibrium was achieved with a 10 Torr partial pressure of Hein the chamber. Before electrical measurements the chamber was evacuatedto a pressure {tilde over ()}≈10⁻⁵ Torr. The data showed littletemperature drift over a typical four hour measurement sweep. Theintrinsic leakage current of the system was measured using a controlsample which had the same substrate and contact pad arrangement as theactual samples, but did not have the organometallic between the pads. Atroom temperature, the leakage current was almost linearly dependent onbias over the range −100 to 100V, and had a maximum value 100 fA. Whilethe ultimate resolution of the current measurement was 10 fA, theleakage current set the minimum resolved conductance {tilde over(≈)}10⁻¹⁵ Ω⁻¹. Constant amplitude RF signals with frequencies,{overscore (f)}, from 0.1 to 5 MHz, were applied to the samples througha dipole antenna at 195K. No attempt was made to optimize the couplingbetween the RF signal and the sample.

[0106] Without RF, the I-V characteristics for one sample at severaltemperatures are shown in FIG. 7. As the temperature was reduced, thelow voltage portion of the curve flattened out and the current becameindistinguishable from the leakage current. Above an applied voltagemagnitude of 6.7±0.6 V, the current increased abruptly. The dataillustrated in FIG. 7 establishes that the monodispersed gold clusterscan produce devices that operate on the basis of the Coulomb blockageeffect. This can be determined from FIG. 7 because one of the curves haszero slope, indicating no current at the applied voltage, i.e., thecluster is blockaded at the particular temperature tested.

[0107] The application of the RF signal introduced steps in the I-Vcharacteristic, as shown in the inset to FIG. 8. FIG. 8 establishes thatan applied external varying signal (the frequency of which is providedby the X axis) actually controls the rate at which electrons movethrough metal clusters made in accordance with the present invention.The current at which these steps occurred was found to be proportionalto the applied signal frequency, as shown in FIG. 8. A least squaresanalysis of the linear current-frequency relationship for the highestcurrent step shown gives a slope 1.59±0.04×10⁻¹⁹C.

[0108] The introduction of plateaus in the patterned sample I-Vcharacteristics is similar to the RF response reported in other Coulombblockade systems. This effect has been attributed to phase locking ofsingle electron tunneling events by the external RF signal. When the nthharmonic of the applied frequency corresponds to the mth harmonic of thefrequency of tunneling in the system, mile, the current becomes lockedto a value I=(n/m)ef. The results obtained suggest that correlatedtunneling is present in the samples.

[0109] The patterned samples had stable I-V characteristics with timeand temperature. Furthermore, as the temperature was raised above about250K the I-V characteristics developed almost linear behavior up to VT.The conductance below V_(T) was activated, with activation energiesE_(A) in the range 30-70 meV. One method to estimate the charging energyfrom the activation energy is to use the argument that the chargingenergy for one island in a infinite two-dimensional array, E_(C){tildeover (≈)}4E_(A). Assuming current suppression requires E_(C)≧10 kT, thesample with the largest activation energy should develop a Coulomb gapbelow ˜300 K. This value is within a factor of 2 of the measuredtemperature at which clear blockade behavior occurs in the patternedsamples. Given the accuracy to which E_(c) is known, the temperaturedependence of the conductance within the Coulomb gap is consistent withthe observation of blockade behavior. Using this value of E_(c), theeffective capacitance of a metal core in the patterned array is3×10⁻¹⁹<C<7×10⁻¹⁹F. These values are close, but larger than theclassical geometric capacitance of an isolated Au₅₅ clusterC=4πεε₀{tilde over (r)}≈2×10⁻¹⁹F, where the dielectric constant of thesurrounding ligand shell ε is expected to be ˜3. The agreement betweenthe two estimates of capacitance supports the notion that the currentsuppression in the metal cluster arrays is due to charging of individualAu₅₅ clusters.

[0110] The non-linear I-V characteristic is similar to that of either aforward biased diode or one-/two-dimensional arrays of ultra small metalislands or tunnel junctions. However, the dependence of the I-Vcharacteristic on the applied RF signal is not consistent withstraightforward diode behavior. Therefore, the data has been analyzed inthe context of an array of ultra small metal islands.

[0111] Several reports have discussed the transport in ordered arrays oftunnel junctions that have tunneling resistances greater than thequantum resistance h/e² and a charging energy significantly above thethermal energy. In this case Coulomb blockade effects introduce athreshold voltage below which current through the array is suppressed.As the applied voltage is increased well beyond threshold, thecurrent-voltage characteristic approaches a linear asymptote with aslope related to the tunnel resistance. With the same temperature andtunnel resistance constraints, Middleton and Wingreen have discussedone- and two-dimensional arrays of maximally disordered normal metalislands where disorder is introduced as random offset charges on eachdot. These authors predict current suppression below a threshold voltageand high bias current I˜(V/V_(T)−1)^(γ). Here, the threshold voltageV_(T) scales with the number of junctions N along the current direction.Analytically γ=1 for one-dimensional systems and 5/3 for infinitetwo-dimensional systems. Numerical simulations of a finitetwo-dimensional array gave γ=2.0±0.2.

[0112] While no effort was made to order samples, the data was analyzedusing both the ordered and the disordered models. The only consistentanalysis was found to be given by the disordered model. In particular,the high bias data did not have the linear asymptote predicted for anordered system, but did scale as expected for a disordered system, asshown in FIG. 9. FIG. 9 also shows that a two-dimensional array so thatsample is propagating through the sample tested along plural parallelpaths. Such an arrangement is important for developing memory storagedevices. The exponent y{tilde over (≈)}1.6 which is closest to theanalytical prediction for an infinite, disordered two-dimensional array.From the analysis the magnitude of V_(T)˜6±1 V which is in goodagreement with that estimated directly from the I-V data.

[0113] The introduction of steps in the I-V characteristics by a RFfield is similar to the RF response reported in other systems. Thiseffect has been attributed to phase locking of single electron tunnelingevents by the external RF signal. If the applied frequency correspondsto a rational fraction multiple of the frequency of tunneling in thesystem, I/e, then the current is locked to a value I=(n/m)ef, where nand m are integers. Therefore, the linear relationships shown in FIG. 6between f and I suggests that correlated tunneling is present in thesamples. The lowest slope observed is best described with n/m=1/5. Forfrequencies up to 3 MHz, the current resolution is insufficient todistinguish between the 1/5 and 1/4 harmonics. However, at higherfrequencies where it should have been possible to distinguish between1/5 and 1/4, the 1/4 step was not observed.

[0114] At temperatures above about 250K, the I-V characteristic wasalmost linear up to V_(T). In this regime the conductance was activated,with activation energies E_(A) in the range 30 to 70 meV for the samplesstudied. Similar activated behavior has been reported for tunneljunction systems. It was argued that for an infinite 2D array thecharging energy for one island E_(C)≈4E_(A). Applying this argument tothe present system, and assuming current suppression requires E_(C)≧10kT, the sample with the largest activation energy should develop aCoulomb gap below about 300 K. This estimate is within a factor of twoof the measured temperature at which clear blockage behavior is seen.Thus, the temperature dependence of the observed current within theCoulomb gap is consistent with the observation of blockade behavior.From the threshold voltage, V_(T)=αNe/C, and this estimate of E_(C), αNis approximately 10. The energy E_(C) can also be estimated if thecapacitance of an island is known. The capacitance of an isolated Au₅₅cluster is C=4πεε₀τ; where τ is the radius of the cluster and ε is thedielectric constant of the surrounding medium. The radius of a Au₅₅ is0.7 nm and the ligand shell is expected to have ε{tilde over (≈)}3,which C≈2×10⁻¹⁹F. The Coulomb charging energy, E_(C)=e²/2C≈340 meV whichis within twenty percent of the maximum value of 4E_(A) found from theactivation data. This result suggests that the current suppression isdue to charging of individual Au₅₅ clusters.

[0115] Given the constraint that steps in the I-V characteristics areonly found when f<0.1(R_(T)C), the fact that steps are seen up to f=5MHz gives the upper limit R_(T)<1x 10¹¹Ω. The differential resistanceobtained from the I-V characteristic well above threshold is anticipatedto be R_(diff)≈(N/M)R_(T), where M is the number of parallel channels.This estimate yields N/M 30. From the sample dimensions and the size ofan individual custer, a close packed array would have N/M˜5. Thisdisparity between the expected and experimentally derived values of theNIM suggests that the full width of the sample is not involved intransport. One explanation for the discrepancy in N/M may be that manyof the gold cores coalesce during sample fabrication so that transportis dominated by individual clusters between larger regions of gold.

Example 6

[0116] This example describes a method for making cluster arrays usingpoly-L-lysine as the scaffold and 11-mercaptoundeconic ligand-stabilizedmetal clusters. Prefabricated electrodes were drop-cast with a 2.2×10⁻⁵mol/l solution of 56,000 amu poly-L-Lysine.HBr in H₂O/CH₃OH. After a20-hour soak in 1% NaOH in nanopure water and a nanopure water rinse,the current-voltage characteristics of the sample were found be becomparable with that of a bare electrode. The polylysine coatedelectrode was then exposed to a drop of 11-mercaptoundeconicligand-stabilized gold clusters in DMSO (about 8 mg/l ml). After about20 minutes, the sample was subjected to a thorough rinse with DMSOfollowed by another rinse in methylene chloride. After correcting forthe leakage current of the bare electrode, the current-voltagecharacteristic of the sample were measured, as shown in FIG. 10.

[0117] A TEM grid was prepared as well using the polylysine scaffold andthe 11-mercaptoundeconic ligand-stabilized gold clusters in DMSO. Thepolylysine solution was drop cast onto TEM grids. A 20-hour soak in 1%NaOH was followed by a nanopure water rinse. The dry TEM grids were thenexposed to a drop of 11-mercaptoundeconic ligand-stabilized goldclusters in DMSO. After about twenty minutes, the grids were thoroughlyrinsed, first using DMSO and then using methylene chloride. Lines ofclusters can be seen in FIG. 11.

Example 7

[0118] This example describes how to make electrical connections tometal cluster arrays. Saw tooth interdigitated array (IDA) goldelectrodes are used and are made using electron beam lithiography. Thegap between saw tooth points in the array will be approximately 200-300Angstroms. An omega-amino alkanethiol will be chemisorbed to the goldsurface and subsequently electrochemically desorbed from one set of theIDA fingers. An omega-NHS-ester alkylthiol will be attached to the bareset of fingers. A precursor to poly-L-lysine will be polymerized fromthe amino-modified fingers toward the NHS-ester fingers where thegrowing end will be captured. The side chains of the poly-L-lysine chainwill be deprotected and treated with carboxy-terminated goldnanoparticles to form the desired one-dimensional array. Gates will beincorporated either under the substrate or as an additional electrodenear (above) the surface of the device.

[0119] The present invention has been described with reference topreferred embodiments. Other embodiments of the invention will beapparent to those skilled in the art from a consideration of thisspecification or practice of the invention disclosed herein. It isintended that the specification and examples be considered as exemplaryonly, with the true scope and spirit of the invention being indicated bythe following claims.

We claim:
 1. An organized array of metal clusters, comprising:monodispersed, ligand-stabilized metal clusters having metal-clusterradiuses of from about 0.7 nm to about 1.8 nm, the metal being selectedfrom the group consisting of Ag, Au, Pt, Pd and mixtures thereof; alinear, biomolecular scaffold, the metal clusters being bonded to thescaffold; and a substrate, the scaffold being coupled to the substrateat predetermined positions to form organized, one-dimensional arraysthat are electrically isolated from one another and having a linearincrease in current above a threshold in applied voltage.
 2. The arrayaccording to claim 1 wherein the scaffold comprises molecules selectedfrom the group consisting of polynucleotides, polypeptides, and mixturesthereof.
 3. The array according to claim 2 wherein the scaffoldcomprises polypeptides capable of forming α helices.